Add null star rating display test case

osu.Game.Tests/Visual/Ranking/TestSceneStarRatingDisplay.cs

@@ -33,6 +33,17 @@ public void TestDisplay()
             });
         }
 
+        [Test]
+        public void TestNullStarRatingDisplay()
+        {
+            AddStep("load null", () => Child = new StarRatingDisplay
+            {
+                Anchor = Anchor.Centre,
+                Origin = Anchor.Centre,
+                Scale = new Vector2(3f),
+            });
+        }
+
         [Test]
         public void TestChangingStarRatingDisplay()
         {